Thin inorganic films formed by CVD have usually been used as interlayer insulating films between wirings in semiconductor devices. As the insulating films for rewiring to be formed on semiconductor devices, thin resin films having a thickness of 20 μm or smaller formed from a photosensitive polyimide resin or photosensitive polybenzoxazole resin are generally used.
In recent years, the operating frequencies of semiconductor devices are increasing so as to cope with the increases in the amount and rate of information communication. In addition, the trend toward size reduction in semiconductor devices for improving the portability and handiness of electronic appliances has resulted in an increased wiring density and a reduced wiring line width. As a result, a decrease in signal voltage due to wiring resistance has come to be pointed out, although this decrease has hitherto been not problematic. On the other hand, a device called a system-on-chip has been proposed. It is a semiconductor chip having all of functions which have hitherto been formed separately on respective semiconductor chips and connected to each other on a circuit board. In this technique, long wirings having a narrow line width are required because different functions are connected to each other on the same chip. It has been proposed to increase the thickness of these wirings on a semiconductor in order to prevent the decrease in signal voltage caused by wiring resistance. In this case, however, a wiring thickness of 10 μm or larger and an insulating layer thickness of 20 μm or larger are expected to be required in the future because the interlayer insulating film should have a thickness sufficiently larger than the wiring thickness.
However, the thicknesses of inorganic films which can be formed by CVD are generally 1 μm or smaller when the productivity thereof is taken into account. Even in the case of the photosensitive polyimides (see documents 1 to 4) and photosensitive polybenzoxazole (see document 5) for use as passivation films in semiconductor devices, it is extremely difficult to deposit a film in a thickness of 20 μm or larger because of a decrease in resolution. Namely, in the case of positive photosensitive materials among those, actinic rays do not reach the interface between the substrate and the resin and, hence, it is difficult to form an image having a thickness of 20 μm or larger through the curing of the photosensitive materials. On the other hand, the negative photosensitive materials heretofore in use can form an image having a thickness of 20 μm or larger. However, use of the negative photosensitive materials has problems that only those parts near the surface to which actinic rays reach are crosslinked and cured and, hence, the resultant pattern has an umbrella shape called a reversed taper, and that because of the insufficient exposure amount, a semi-cured material remains as a residue after development.
Under these circumstances, a photosensitive polyimide of the dissolution control type employing a 1,4-dihydropyridine derivative as a photosensitizer has recently been proposed (see documents 6 and 7). This photosensitive polyimide is free from the problem in existing negative materials that a residue of the photosensitive material remains after development. Furthermore, the disadvantage that the photosensitive polyimide gives a pattern having a taper shape can be mitigated to some degree by conducing a heating (post-exposure heating) treatment after exposure and before development. In particular, incorporation of polyethylene glycol as a development accelerator into the photosensitive polyimide has enabled the photosensitive material to give an image which has a sufficient heat treatment margin even when the thickness thereof after cure is as large as 20 μm or more (see documents 7 and 8). However, since this photosensitive material tends to give patterns having a normal taper shape in contrast to negative materials heretofore in use, the resolution of such patterns comprising a thick film is about 40–50 μm at the most.
Other photosensitive materials which have been proposed include a photosensitive epoxy resin. However, this epoxy resin has a higher permittivity and a higher dielectric loss than the photosensitive materials described above and these properties result in a reduced rather than increased signal voltage, leading to a decrease in SN ratio.                Document 1: JP-A-49-011541        Document 2: JP-A-50-140922        Document 3: JP-A-54-145794        Document 4: JP-A-56-038038        Document 5: JP-A-07-281441        Document 6: JP-A-06-075376        Document 7: JP-A-07-271034        Document 8: JP-A-2002-148804        